Locomotives include various systems for cranking the engine in order to assist engine starting. The starting systems may utilize an engine starter motor, or other engine propulsion components. In one example, the propulsion system includes a battery that provides direct current (DC) power to an inverter that converts the DC power into a controlled frequency alternating current (AC) power. The AC power is then supplied to three-phase stator windings of an alternator that generates rotation of a rotor, which when coupled with the crankshaft of the engine, rotates the crankshaft for engine starting.
In U.S. Pat. No. 6,023,137, one such propulsion system is disclosed. In this example, a traction inverter converts battery DC to AC for alternator operation. After cranking, the traction inverter is reconnected to an AC traction motor that provides traction for the locomotive during engine running operation. The traction inverter is connected in series to the stator field winding and a field-shunting resistor. The shunting action is enabled only when a predetermined amount of time has elapsed after the commencement of cranking (e.g., engine speed is at least greater than 30 rpm), so that alternator torque can be readjusted to a higher value during a later part of the cranking. Load current is limited by battery voltage, back electromotive force (EMF) of the armature voltage, and circuit impedance, including that of the field winding. For example, the above system may perform cranking for battery voltages between 50V and 64V by effectively providing two discrete levels of alternator torque adjustment.
However, the inventors herein have recognized a potential issue with such an approach. For example, battery voltage and cranking torque specifications may vary significantly with the type of battery system, engine ratings (e.g., HP rating, rated engine speed), number of cylinders, fuel types, engine operating conditions, alternator design (e.g., armature inductance, resistance, field characteristics, and torque generation characteristics), environmental conditions, and/or system aging. Thus, even with two levels of alternator torque adjustment, degraded starting may occur under various situations, if the same configuration is applied to locomotives with different engine ratings, battery voltage, etc. Further, the limitation in alternator output adjustment may result in numerous starter electronics configurations, which can increase manufacturing complexity, repair complexity, etc.